Method for the production of glass fiber-reinforced gypsum sheets and gypsum board formed therefrom

ABSTRACT

A fiber-reinforced cementitious sheet is formed by first co-mixing in an air current reinforcing fibers such as glass fibers, and cementitious mineral materials in finely particulate form such as calcium sulfate hemihydrate, both in substantially dry form, depositing the mixture on a moving foraminous surface by means of the air current to form a sheet, applying water as by spraying in at least an amount which is stoichiometrically sufficient to hydrate the calcium sulfate hemihydrate to the dihydrate form and to provide the necessary degree of plasticity to the mixture, densifying the sheet by compression, and setting and drying the sheet. A pair of sheets may be utilized as face sheets and, prior to setting, combined with a core formed of for example a calcium sulfate hemihydrate slurry, and the sheets and core then set and dried to form a paper-free gypsum board having excellent strength, surface hardness, and fire-resistant properties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to cementitious construction sheets, suchas gypsum wallboard and more particularly refers to a new method forforming paper-free glass fiber-reinforced cementitious sheets such asgypsum and utilizing the sheets with a standard cementitious slurry coreto form paper-free wallboard, and to the products formed thereby.

2. Description of the Prior Art

Wall panels or wallboards made of rehydrated stucco conventionallycomprise a gypsum core of uniform density sandwiched between two papercover sheets. Such panels can be mass produced and erected soinexpensively that they have largely replaced prior building techniquesusing wood panels or plaster. As the usage of such wallboard hasexpanded, however, specialty uses such as walls in high-rise officebuildings and apartments have placed a premium on certain properties.Specifically, shaft walls used, for example, as elevator shafts, airreturn shafts, and stairwells are subject in some instances to verystrict fire regulations. Thus there is a trend in municipal fire codestowards requiring a 0-0-0 fire rating for the exposed surface ofelevator shafts, that is, having zero flame spread, zero smoke, and zerotoxic gas generation. It has not been possible to achieve such ratingsas long as paper-covered wallboard is used, due to the combustibility orat least the smoke-generation capability, of the paper cover sheets.Such paper cover sheets are further troublesome in that they appreciablydelay the drying time of the board during its manufacture.

A further problem characteristic of certain elevator shafts is that windloading causes constant flexing of the wallboard. Thus, when used insuch walls, the wallboard must have good flexural strength--a physicalproperty not exhibited by rehydrated stucco alone due to its low modulusof rupture.

Some presently manufactured wallboard does include various ingredientswhich impart fire resistance to the board. For example, glass fibers onthe order of one-half inch in length have been incorporated throughoutthe core of paper-covered gypsum wallboard used to line elevator shaftwalls, on a weight percent basis of about 0.25% of the weight of theboard. However, such fibers are not long enough to contributesignificantly to the flexural strength of the board, as theconcentration is insufficient, and at that length, the fibers' pull-outstrength is insufficient.

Numerous methods have been developed through the years to combine glassfibers and gypsum in order to produce various articles such asreinforced gypsum wallboard. It was early realized that in order todevelop good strength properties the glass fibers must be evenlydispersed and a gypsum matrix must be used which has high strength.

When glass fibers and gypsum are mixed in the form of an aqueous slurry,the length and amount of fibers which may be added are limited in orderto prevent balling of the fiber during mixing. Excess water is alsorequired to make the slurry sufficiently fluid to be formed into thedesired article. The use of excess water reduces the strength of thegypsum matrix. This is disclosed in British Pat. No. 1,204,541. In orderto remove excess water, a means of applying suction and pressure to theformed board has been developed. This process is cumbersome and costlyand not well adapted to high speed production.

A similar approach is illustrated in New Zealand Pat. No. 155,679, whichteaches a gypsum panel constructed with glass fibers of various lengths,dispersed generally throughout the rehydrated stucco. Such aconstruction has eliminated the need for a paper cover sheet. However,the process of making such panels is difficult, time-consuming, andinvolves the use of a large proportion of glass fibers, inasmuch as theyare distributed more or less uniformly throughout the board or panel.

Another prior art process for producing glass fiber-containing gypsumboard involves co-spraying discontinuous glass fibers and gypsum in anaqueous slurry onto a moving belt. Special low water demand plaster isused, or suction is used to remove the excess water. The majordisadvantages of this process are that the fiber strands are notdispersed into individual filaments of which they are formed, thusreducing the efficiency of the fiber as a reinforcing agent, and thatthe fibers are not sufficiently mixed with the plaster.

Glass fiber mats have also been used to reinforce gypsum. These mats maybe in the form of continuous or discontinuous, random oriented fiber, oras woven mats. The mats are saturated with gypsum using various meansand methods. One method is described in Canadian Pat. No. 993,773. Themats are fabricated with glass fiber strands each consisting of aplurality of glass fiber filaments. The gypsum slurry does not saturatethe fiber strand and therefore the reinforcing efficiency of the fiberis reduced. Special low water demand gypsum such as alpha-calciumsulfate hemihydrate must be used to obtain high gypsum strength.Alternatively, excess water must be removed by means of suction.

In copending U.S. application Ser. No. 666,539, filed Mar. 15, 1976, andwhich is a continuation of U.S. Ser. No. 592,960 filed July 3, 1975, nowabandoned, which is in turn a continuation of U.S. Ser. No. 415,038 alsonow abandoned, there is disclosed a reinforced gypsum board comprising acore of relatively low density being free of glass fibers, and providedon both surfaces thereof with skin layers formed of rehydrated gypsumhaving glass fibers dispersed therein. Preferably the outer layers areformed of gypsum having a higher density than the gypsum of the core.The preferred material for making the higher density outer layers isalpha calcium sulfate hemihydrate. Each outer layer is preferably bondedto the core by intercrystalline growth during the forming process. Informing the product, an aqueous stucco slurry containing glass fibers isdeposited on a moving flat surface to form one outer layer, a lowdensity aqueous slurry of calcium sulfate hemihydrate is deposited overthe first outer layer to form a core, and a second outer layer of anaqueous slurry of calcium sulfate hemihydrate containing glass fibers isdeposited over the core layer. No paper cover sheets are applied to theouter surfaces of the board. The resulting gypsum wallboard isrelatively light since a low density gypsum is used for one core, and isvery strong due to the glass fibers contained in the outer layers. Afurther advantage of the product is that there are no cover sheets tohinder the evaporation of excess water in the drying kiln, therebyreducing the processing time and reducing the cost of fuel for drying.The resulting board also has a high flexural strength. Moreover, due tothe central plane of symmetry of the board the strength is the sameregardless of the face on which it is measured. Because a relatively lowdensity gypsum is used for the core and relatively high density gypsumis used for the thin outer skins, the increase in strength is achievedwithout a concommitant increase in overall weight. The product is statedto be excellent for use in elevator shaft wall and for use in elevatorair return shafts and stairwells where the product is subject to verystrict fire regulations, and must undergo considerable flexing due towind loading.

In U.S. Pat. No. 3,682,670, there is disclosed a process for preparingfiber-containing plaster products wherein glass wool and/or rock wool iscarded, plaster powder added to the fibers as they are carded to providea dry composition, and the dry mixture is then introduced into an excessof water to form a slurry which is subsequently cast in the form ofboards. However, this process is somewhat deficient in that a largeexcess of water is introduced which must be subsequently removed bydrying. Additionally, the carding process does not produce a gooduniform mix of the fibers and plaster.

In U.S. Pat. No. 1,862,318, a method is disclosed for producing plasterboard containing cotton linters which comprises first depositing a layerof gypsum on a moving belt subsequently depositing the cotton lintersthereover while carding, and finally sprinkling water over the layerthus formed and compressing the layer by rolling. In this method,because the gypsum and fibers are not premixed but, merely sprinkledonto the belt, a uniform layer is not produced.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a novelmethod for preparing cementitious construction sheets having reinforcingfibers dispersed therein.

It is a further object to provide a cementitious construction sheet ofthe type described which utilizes a minumum of water for setting thesheet.

It is a further object to provide a method for forming constructionwallboard free of paper cover sheets by combining a pair of cementitiousconstruction sheets of the type described together with a settablecementitious core.

It is still further an object to provide a method as described whereinonly sufficient water is added to set the cementitious material of theouter layers to provide adequate setting without the need tosubsequently evaporate a large excess of water during the drying stage.

These and other objects, advantages and functions of the invention willbe apparent upon reference to the specification and to the attacheddrawings illustrating the preferred embodiments of the invention, inwhich like parts are identified by like reference symbols in each of theviews.

According to the invention, thin layers of a cementitious material suchas rehydrated gypsum containing fibers such as glass fibers are preparedby mixing together in a flowing stream or current of gas, such as air, asubstantially dry water-settable cementitious material, such as calciumsulfate hemihydrate, and fibers, such as glass fibers. The dry mixtureis deposited by means of the flowing gas stream or current such as anair stream or current onto the surface of a moving belt which ispreferably foraminous or perforated, such as a screen, to permit the gasto pass therethrough while depositing the solid material in the form ofa sheet. Subsequently water is sprayed onto the dry sheet in an amountsufficient to provide adequate plasticity and to completely set thecalcium sulfate hemihydrate. Water is used slightly in excess of thestoichiometric amount for complete setting and for adequate plasticity,but without providing a large excess of water. The sheet thus formed ispassed between compression rolls. An aqueous slurry of calcium sulfatehemihydrate may then be deposited over the compressed layer and a secondcompressed layer of gypsum identical to the bottom layer placed over theslurry. The gypsum of both the outer layers and of the core is thenpermitted to set and is subsequently dried in a kiln. The resultingproduct is fire-resistant, strong, and relatively light.

Throughout the specification and claims, wherever the term "stucco" isutilized, it is intended to have the meaning attributed to it by thoseskilled in the gypsum art. As used herein, the term "stucco" denotescalcined gypsum or calcium sulfate hemihydrate, either in the alpha orin the beta form.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a side elevational diagrammatic view of an apparatus used toproduce glass fiber-reinforced gypsum sheets according to the invention.

FIG. 2 is an enlarged cross-sectional view of a portion of the apparatusof FIG. 1.

FIG. 3 is a side elevational diagrammatic view showing the apparatusutilized for producing gypsum board having glass fiber-reinforced outerlayers.

FIG. 4 is a cross-sectional view of glass fiber-reinforced gypsum boardproduced according to the method of the invention.

FIG. 5 is a side elevational diagrammatic view showing apparatus forincreasing the density of the formed sheet and having means forembossing one surface of the sheet, and

FIG. 6 is an end view of a mold which may be utilized to densify andshape the glass fiber-reinforced gypsum sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an apparatus is shown for carrying out the processof the present invention for forming a glass fiber-reinforced gypsumsheet, and comprises generally an apparatus 10 for preparing the glassfiber and mixing the fibers with stucco or calcium sulfate hemihydrate,and depositing the mixture on a screen by means of an air current; andan apparatus 11 for conveying the layer of glass fibers and stucco,applying a water spray thereto, and rolling the wet layer to increaseits density.

The glass fiber preparation and stucco mixing apparatus 10 is generallya commercial apparatus produced by the Rando Machine Corporation,Macedon, N.Y., and is marketed under the Registered Trademark "RANDO".The apparatus is made up of several sections including a prefeeder 12,an opener blender 13, a feeder 14, and a mixer and air depositingapparatus 15.

The prefeeder 12 comprises a housing 16, a floor apron 17 comprised of apair of rolls 18 and 19, and an endless belt 20 mounted thereon. Theapparatus further comprises an elevating apron 24 comprising a pair ofrolls 25 and 26 and a barbed endless belt 27 mounted thereon. A stripperapron 28 is horizontally mounted in the upper portion of the chamber andcomprises a pair of rolls 29 and 30 having a barbed endless belt 31mounted thereon. A doffer roll 32 is mounted to cooperate with elevatingapron 24 to remove glass fibers from the surface thereof. An electricmotor 33 is operatively connected to drive the various endless belts. Ananti-static spray nozzle 34 and associated equipment (not shown) areprovided to prevent the formation of static charges which might causethe glass fibers to clump.

The fiber opener and blender 13 comprises a housing 36 having therein afloor apron 37 comprised of rolls 38, 39 and 40 and an endless belt 41mounted thereover. Barbed worker rolls 42 and 43 cooperate with astripper roll 44 to open up the cut glass strands of the clippings andseparate them into individual fibers. A hopper cover 45 prevents theopen fibers from escaping. A main cylinder 46 cooperates with smallworker rolls 47 and small stripper rolls 48 to further separate theglass fibers and transport the fibers to an air brush 49. Motors 50 areoperatively connected to the various rolls and provide motor power. Ananti-static spray nozzle 51 and associated equipment (not shown)prevents static charges from building up.

The feeder 14 comprises a fiber separator 52, and a floor apron 53comprised of rolls 54 and 55 and having an endless belt 56 mountedthereon. A vertically positioned elevating apron 57 comprises a pair ofrolls 58 and 59 and an endless conveying belt 60 mounted thereover,conveying the fibers to an upper portion of the chamber which contains ahorizontally mounted stripper apron 61 mounted on rolls 62 and 63 andhaving a barbed endless belt 64 mounted thereon.

As shown in greater detail in FIG. 2, an air bridge 65 connects to afeed mat condenser screen 66. A roll conveyor 68 cooperates with a feedplate 69 and feed rolls 70 to convey the fibers forward. A nose bar 71,lickerin 72 and saber roll 73 convey the fibers into a venturi chamberor duct 74 where a feeder 77 feeds stucco or other cementitiousmaterials 78 into the venturi chamber 74 where the stucco is intimatelymixed with the fibers by the air current passing through the venturichamber 74. A condenser screen 79 in the form of an endless belt mountedon rolls is provided for collecting the air-blown mixture of glassfibers and cementitious material and conveys the formed web 87 to watertreatment conveyor portion 83. Air current for mixing the stucco andfiber in the venturi chamber 74 and depositing the mixture on thecondenser screen 79 is provided by blowers 80 and 81. The air isconveyed to a dust collector 82 for removing stucco and fibers which mayhave passed through the condenser screen 79 by means of a duct 95.

The web passes to a water treatment conveyor system 83 comprising rolls84 and 85, and an endless belt 86 mounted thereover. A water spray 92 isprovided by means of a water duct 90 and a nozzle 91. The water treatedweb 87 then passes through densification rolls 93 to form a densifiedfiber-reinforced gypsum sheet 94.

In operation chopped glass fiber strands are introduced into theprefeeder 12 where, after preliminary processing, they are introducedinto the fiber opener and blender 13. Here the various barbed rolls openup the glass fiber strands and free the individual glass fibers. Thefibers are then conveyed to the feeder 14 where they are ultimatelyintroduced into the venturi duct 74. The cementitious mixture such asstucco 78 is fed from the feeder 77 into the venturi chamber 74, whereit is intimately mixed by the air stream with the glass fibers. Themixture is then desposited on the condenser screen 79. A vacuum ismaintained below the screen for directing the air stream through thescreen to assist in directing the stucco and glass fibers towards thesurface. The deposited web comprising glass fibers and stucco is thenconveyed by the condenser screen 79 to the water treatment conveyorsystem 83, where water is sprayed over the web in an amount justsufficient to permit the stucco to become hydrated and to form setgypsum or calcium sulfate dihydrate. Only a very slight excess of waterif any need to be used to provide suitable plasticity. The water-treatedweb then passes through densification rollers and subsequently sets to avery hard dense sheet of glass fiber-reinforced gypsum, and issubsequently dried.

Referring to FIG. 3, an apparatus is shown which is used to apply twoglass fiber-reinforced sheets 94 to the upper and lower surfaces of adeposited gypsum slurry. The apparatus comprises a moving endless belt100 mounted on rolls 101, 102 and 103. Additionally a compression roll104 is mounted for compressing the two glass fiber-reinforced sheets 94against the gypsum slurry.

In operation two glass fiber-reinforced sheets 94 as formed by theapparatus of FIG. 1 are introduced into the apparatus, one sheet beingsupported on the moving belt 100. A conventional stucco slurry 98 ispoured onto the lower sheet from a slurry mixer 99 and then the uppersheet 94 compressed against the slurry 98 by means of the rolls 102 and104. The three layered structure is then permitted to set, and theexcess water subsequently evaporated in a kiln.

Referring to FIG. 4, a portion of a three layered structure 105 is shownin cross-section and comprises a core 106 and outer glassfiber-reinforced layers or sheets 107 and 108 similar to the sheet 94shown in FIG. 3. In accordance with one aspect of the invention, thecore 106 has a relatively lower density and is substantially free ofglass fibers, while the reinforced layers or sheets 107 and 108 haveglass fibers 109 dispersed throughout, and have a relatively higherdensity. In a preferred form the core is made of foamed beta calciumsulfate hemihydrate, while the outer layers 107 and 108 contain glassfibers and are made of either alpha or beta calcium sulfate hemihdyrate.As disclosed above, the outer layers are rolled to increase theirdensity. The outer surfaces 110 and 111 of the outer layers 107 and 108,respectively, are free of paper cover sheets, since, as a result of thedensification rolling step, they acquire a very hard and strong surface.The entire board is highly fire-resistant and smoke-resistant,relatively light, and has excellent flexural strength.

FIG. 5 illustrates a subassembly for producing an embossed or texturedsurface on the outer surface of the sheet. The wetted sheet 87 is firstpassed through the densification rolls 93 and embossing roll 112 havinga pattern on its surface operating against a standard roll 93. Theresulting sheet 94 has an embossed pattern on the surface which is tobecome the outer surface of a completed gypsum board.

FIG. 6 illustrates a means for forming the finished sheet 87 to anydesirable form. The sheet 87 is placed between two complimentary moldforms 113 and 114 supported by press plattens 115 and 116. The assemblyis placed in a press and the sheet 87 is wetted and molded to form andsubsequently permitted to cure and to be dried.

EXAMPLES

The following examples are provided for illustrative purposes only andare not intended to be limiting.

EXAMPLE 1. Preparation of Glass Fiber-reinforced Gypsum Sheet.

Glass fiber strands cut to one half inch in length were processed toseparate the strands into individual fibers having a diameter of about0.00025 inch. The glass fibers were mixed together in a moving aircurrent with beta calcium sulfate hemihydrate in a proportion whereinthe glass fiber was present in an amount of ten percent (10% ) by weightof the total glass and hemihydrate. The glass fibers and hemihydratewere mixed together in a moving air stream and the mixture deposited ona moving screen belt. The dry deposited sheet was wetted by a waterspray wherein approximately thirty-five (35) pounds of water per onehundred (100) pounds of calcium sulfate hemihydrate were utilized. Thesheet was densified by passing between two sets of rolls having a nippressure of 118 lb/linear inch. After densification, the hemihydrate ofthe sheet was allowed to hydrate or set. The sheet was then dried to aconstant weight at 110° F. The density of the sheet was about 75 lbs.per cubic foot and the thickness 0.030 inch.

Specimens were prepared from the sheet for tensile strength evaluation.The prepared specimens were 12 inches long, and 3 inches wide at eachend. They were necked down to 2 inches in width for an 8 inch lengthstarting about 2 inches from each end of the specimen. The specimenswere conditioned at 75° F. and fifty percent (50% ) relative humiditybefore testing. An Instron testing machine was used. The specimens werepositioned in the machine fixture so that the load was applied to the 2inch wide area with a elongation of 0.109 inch was required to causefailure of the material. A tensile load of about 163 pounds with anelongation of 0.074 inch was required to cause failure of a specimen ofregular wallboard paper prepared in the same manner.

EXAMPLE Preparation of Gypsum Wallboard Panels

A pair of glass fiber-reinforced gypsum sheets were prepared asdescribed in Example 1. Immediately after densification of the gypsumsheets but before setting of the stucco had taken place, one sheet wasplaced in the bottom of a mold. A standard beta calcium sulfatehemihydrate wallboard slurry having conventional additives, including afoaming agent was poured onto the top of the gypsum sheet. The secondsheet was then placed on top of the slurry and the structure wasconsolidated between rolls. The finished panel was about one-half inchthick. The dry density of the core was 45.4 pounds per cubic foot.Conventional accelerator and retarder materials were used in both theglass fiber containing gypsum sheets and the core slurry, and thecompositions were so adjusted as to permit substantially simultaneoushydration or setting of the calcium sulfate hemihydrate in both the coreand the outer sheets. In this manner a very good bond resulted betweenthe layers as a result of the inner growth of crystals at the layerinterfaces. After hydration or setting was complete, the panel wasdried.

Specimens measuring 6×14 inches were cut from the panel. The specimenswere conditioned at 75° F. and 50% relative humidity before testing. Thebending or transfer strength was then determined by placing the specimenonto supports spaced 12 inches apart. A load was then applied from aboveat the center of the span causing the material to bend until failure. Aload of 130 pounds causing a deflection of about 0.384 inch was appliedbefore the specimen failed.

EXAMPLE 3 Comparative Example.

A conventional gypsum panel having standard paper cover sheets but noglass fiber reinforcements was prepared. The stucco was set and dried asin the pevious examples, and tested for transfer strength. With theconventional panel a bending load of only 108 pounds with a deflectionof 0.234 inch was required to cause failure of the panel. The load testwas made in the strong direction of the panel.

EXAMPLE 4 Testing under High Humidity.

Panels as made in Examples 2 and 3 were tested for resistance todeflection under humid conditions. The 12×24 inch specimens in each casewere supported across the 12 inch end and maintained at 90° F. and 90%relative humidity conditions. After ten (10) days the deflection or sagof each panel was tested. The panel prepared with glass fiber-reinforcedouter sheets showed a deflection of 0.075 inch, whereas the conventionalpanel having paper cover sheets had deflected about 0.195.

Although the invention has been described above in relation to the useof beta calcium sulfate hemihydrate as the settable cementitiousmaterial, the alpha form may also be used and for some purposes mayyield superior products. Moreover, other settable cementitious materialsmay be used, such as a mixture of alpha calcium sulfate hemihydrate andcement, conventional hydraulic cement such as portland cement, magnesiumoxychloride, and related materials. High early strength portland cementsmay also be used. It is only necessary to use a material which iscompatible with the particular fiber used. For example, if portlandcement is used, an alkali-resistant glass fiber must be used.

The invention has also been described in terms of its use with glassfibers for reinforcement. However, other fibers such as polyester,acrylic, nylon, carbon, rock wool, asbestos fiber, etc., may be used.The fiber lengths may be from one-half to six inches, preferablyone-half to two inches. Where glass fiber strands are utilized, thefibers should first be opened up in a machine 13 such as that describedabove and then conveyed to the fiber feeder 14 which meters the fiberinto the air stream of the web former. The feeder 77 then meters thecementitious materials such as calcium sulfate hemihydrate into the airstream. With the apparatus shown and described, it is convenient tointroduce the glass fibers into the air stream first, and then tointroduce the calcium sulfate hemihydrate downstream from theintroduction of the glass fibers. Alternatively, the calcium sulfatehemihydrate may be introduced into the air stream first followed by theglass fiber downstream. In another satisfactory variation, the glassfibers and the calcium sulfate hemihydrate may be simultaneouslyintroduced at the same position of the air stream. The importantconsideration is that the fibers and the calcium sulfate hemihydrate beuniformly blended within the air stream before thay are deposited ontothe traveling condenser screen 79 where the web is formed.

Patterns such as wood grain, brick, etc., may be embossed into thesurface of the sheet. The embossing is completed before hydration of thehemihydrate takes place. Subsequently the material is hydrated anddried. If the embossed sheet is to be utilized for making a laminatedboard, after embossing the sheet is applied to a calcium sulfatehemihydrate slurry for forming a core, and the sheet and core thenhydrated and dried together.

Densification of the glass fiber-reinforced sheet may be accomplished bymeans other than rolls. As shown in FIG. 6, the dry sheet of glass fiberand stucco may be removed from the collector screen, placed onto thebase of a mold and sprayed with sufficient water to hydrate the stuccoand to render the sheet somewhat plastic. A matching section of the moldis then placed on top of the wetted sheet, the assembly placed betweenpress plattens 115 and 116 and a force is applied to compact the sheets.A force of about 50 pounds per square inch is satisfactory, but theforce may be varied to develop the desired density. With this processthe sheet is simultaneously densified and molded into various desiredshapes.

The process of the present invention has many advantages over prior artprocesses. First, because the fibers and dry cementitious material ismixed in an air stream, excellent mixing is accomplished without anyclumping of the fibers. The problem of separating the strands or tuftsof fiber into individual filaments is overcome by processing the fiberbefore it is mixed with the stucco and then suspending the filaments inan air stream. When the stucco is then fed into the air stream, thoroughblending of the fiber and gypsum is accomplished without any clumping.

Because the sheet of glass fiber and stucco is formed in the dry state,it is unnecessary to use a large amount of water to fluidize thematerial. Only sufficient water need be utilized to stoichiometricallyhydrate the stucco and to make it sufficiently plastic so that it can bedensified. The ability to use varying amounts of water and to densifythe sprayed sheet to varying degrees permits the production of glassfiber-reinforced gypsum sheet with a wide range of properties. If hightensile strengths are desired, the gypsum matrix sheet should bedensified to a high degree. However, for some applications as forexample the fabrication of art objects, a high tensile strength is notrequired but a more desirable low density article may be produced whichhas good impact and crack-resistance. To achieve this, higher amounts ofwater and lower densification pressure may be used. In the fabricationof calcium sulfate dihydrate sheets, additives are commonly added tocontrol the rate of hydration. Additionally additives to increase theplasticity of the mix may be used. Polymers may also be used to increasethe toughness of the article or to improve painting properties. All theconventional additives may be used in the present process to the extentthat they are compatible with the particular fiber used. The additivesmay be blended with the dry stucco, or, those which are water soluble,may be added to the water used to wet the sheet.

In producing products according to the invention, various glass fiberparameters may be utilized. For example, the amounts of glass fiber usedmay be from 3-25% based on the weight of the dry formed sheet. Thepreferred range is from 6-10%. The glass fiber length may be from onehalf to six inches. A preferred length is from one to three inches.Glass fibers having diameters of 0.00023 to 0.007 inch may be utilized.A preferred range is 0.00025 to 0.00038 inch. The water to gypsumhemihydrate ratio may be from 0.25 to 0.60 by weight. A preferred ratiois 0.30 to 0.45.

The present invention has many advantages over processes disclosed inthe prior art and over products produced by these processes. First, byintroducing fibers which have been treated to transform them intoindividual fibers, and introducing the fibers into a moving air streamcauses an excellent separation of the fibers and prevents clumping. Theintroduction of the cementitious setting material into the moving airstream results in excellent and uniform mixing of the fibers andcementitious material. Finally, the co-deposition of the mixturecontained in the air stream on a moving foraminous screen results in aweb of fibers and cementitious material in very uniform distribution.One advantage is that because of the vacuum applied at the leeward sideof the stream, the fibers may be deposited with an orientation whereinthey are somewhat oblique to the plane of the web, and this causesvertical interweaving of the fibers to produce a material of greaterperpendicular tensile strength. Additionally, the step of spraying wateronto the moving web in an amount which is not materially greater thanthe stoichiometic amount conserves on energy required for drying thematerial and also results in a material of greater strength becauselarge excesses of water are not required to provide the necessaryconventional fluidity of the mixture, since the sheet is dry formed andcan be densified or further formed with the use of relatively smallamounts of water. Additionally, when two sheets according to theinvention are bonded to a gypsum core, no starch need be utilized forbonding.

It is to be understood that the invention is not to be limited to theexact details of composition, materials or operation shown or described,as obvious modifications and equivalents will be apparent to one skilledin the art.

What is claimed is:
 1. A method for preparing a cementitiousconstruction sheet having reinforcing fibers dispersed therein, whichcomprises:(a) mixing together in a moving air current generated by anair blower a substantially dry water-settable calcium sulfatehemihydrate in finely particulate form with reinforcing glass fibers,(b) providing a foraminous moving surface, (c) depositing the mixture ofcalcium sulfate hemihydrate and glass fibers on said foraminous surfaceto form a sheet, (d) applying water over the sheet, (e) permitting thecalcium sulfate hemihydrate to set, and (f) drying said sheet to removeany excess water.
 2. A method according to claim 1, wherein said wateris applied by spraying.
 3. A method according to claim 2, wherein avacuum is applied below said moving foraminous surface to assist indirecting said cementitious material and said fibers toward saidsurface.
 4. A method according to claim 2, wherein said sheet isdensified by compression prior to setting.
 5. A method according toclaim 2, wherein the amount of water is not substantially greater thanthe stoichiometric amount necessary to hydrate and set said cementitiousmaterial.
 6. A method according to claim 1, wherein said calcium sulfatehemihydrate is in the beta form.
 7. A method according to claim 1,wherein said calcium sulfate hemihydrate is in the alpha form.
 8. Amethod according to claim 2, wherein said cementitious material is amixture of alpha calcium sulfate hemihydrate and hydraulic cement.
 9. Amethod according to claim 2, wherein said cementitious material is ahydraulic cement.
 10. A method according to claim 2, wherein saidcementitious material is very high early strength hydraulic cement. 11.A method according to claim 2, wherein the thickness of said sheet isfrom about 0.010 inch to about 0.250 inch.
 12. A method according toclaim 2, wherein the weight of fibers is from about 3% to about 25%based on the weight of the dry formed sheet.
 13. A method according toclaim 2, wherein the weight of fibers is from about 6% to about 10%based on the weight of the dry formed sheet.
 14. A method according toclaim 1, wherein said glass fibers are formed by chopping glass fiberstrands and subsequently opening the chopped strands into individualfibers.
 15. A method according to claim 5, wherein said sheet isdensified by passing it between compression rolls.
 16. A methodaccording to claim 2, wherein a pattern is applied to one surface byembossing prior to setting said cementitious material.
 17. Acementitious construction sheet having reinforcing fibers dispersedtherein, said sheet being the product formed by the method whichcomprises:(a) mixing together in a moving air current generated by anair blower a substantially dry water-settable calcium sulfatehemihydrate in finely particulate form with reinforcing glass fibers,(b) providing a foraminous moving surface, (c) depositing the mixture ofcalcium sulfate hemihydrate and glass fibers on said foraminous surfaceto form a sheet, (d) applying water over the sheet, (e) permitting thecalcium sulfate hemihydrate to set, and (f) drying said sheet to removeany excess water.
 18. A sheet according to claim 17, wherein said wateris applied by spraying.
 19. A sheet according to claim 18, wherein avacuum is applied below said moving foraminous surface to assist indirecting said cementitious material and said fibers toward saidsurface.
 20. A sheet according to claim 18, wherein said sheet isdensified by compression prior to setting.
 21. A sheet according toclaim 18, wherein the amount of water is not substantially greater thatthe stoichiometric amount necessary to hydrate and set said cementitiousmaterial.
 22. A sheet according to claim 17, wherein said calciumsulfate hemihydrate is in the beta form.
 23. A sheet according to claim17, wherein said hemihydrate is in the alpha form.
 24. A sheet accordingto claim 18, wherein said cementitious material is a mixture of alphacalcium sulfate hemihydrate and hydraulic cement.
 25. A sheet accordingto claim 18, wherein said cementitious material is a hydraulic cement.26. A sheet according to claim 18, wherein said cementitious material isvery high early strength hydraulic cement.
 27. A sheet according toclaim 18, wherein the thickness of said sheet is from about 0.010 inchto about 0.250 inch.
 28. A sheet according to claim 18, wherein theweight of fibers is from about 3% to about 25% based on the weight ofthe dry formed sheet.
 29. A sheet according to claim 18, wherein theweight of fibers is from about 6% to about 10% based on the weight ofthe dry formed sheet.
 30. A sheet according to claim 35, wherein saidglass fibers are formed by chopping glass fiber strands and subsequentlyopening the chopped strands into individual fibers.
 31. A sheetaccording to claim 20, wherein said sheet is densified by passing itbetween compression rolls.
 32. A sheet according to claim 18, wherein apattern is applied to one surface by embossing prior to setting saidcementitious material.